php dynamic stylesheet & wp-blog-header.php

I, I would like to have a dynamic stylesheet (php) which loads different css files depending of certain conditions.

wp_enqueue_style( 'yclads-css', yclads_get_theme_file_url('style.php') );

in my style.php :

<?php
require('../../../../wp-blog-header.php');
header('Content-type: text/css');

?>
body{
	background-color:#000;
}
@import url('_inc/css/shared.css');

<?php if(!defined('BP_VERSION')){//load styles for WP ?>
@import url('_inc/css/screen-wp.css');
<?php }else{ //load styles for BP ?>
@import url('_inc/css/screen-bp.css');
<?php
}
?>

I need to load wp-blog-header.php to access the WP functions.
But with this line, the css is broken : here, the body background will not be black. If I comment the line, it is black.
When I open the source file from firebug; the output seems ok :

body{
	background-color:#000;
}
@import url('_inc/css/shared.css');

@import url('_inc/css/screen-bp.css');

Any idea ?